GENEVA (AP) – The World Health Organization said Thursday that up to 2 billion people could be infected by swine flu if the current outbreak turns into a pandemic. The agency said a pandemic typically lasts two years.

WHO flu chief Keiji Fukuda said the number wasn’t a prediction, but that experience with flu pandemics showed one-third of the world’s population gets infected.

“If we do move into a pandemic then our expectation is that we will see a large number of people infected worldwide,” Fukuda said. “If you look at past pandemics, it would be a reasonable estimate to say perhaps a third of the world’s population would get infected with this virus.”

In Mexico, which has had the most cases, high schools and universities opened for the first time in two weeks as the country’s top health official insisted the epidemic is on the decline. All students were checked for swine flu symptoms and some were sent home.